It has long been hypothesized that synaptic rearrangements in the developing nervous system are mediated by competition between axons for a limited supply of target-derived neurotrophic molecules. However, there has been little direct evidence indicating local maintenance of synapses by retrogradely acting factors. One site where target-derived factors might be playing a pivotal role is at the developing neuromuscular junction where synapses from all but one axon are eliminated in early postnatal life through a competitive mechanism that is little understood. The developing neuromuscular junction is a powerful system in which to study synaptic competition as boutons from competing axons can be visualized and assessed physiologically as they are eliminated over the first two postnatal weeks. Glial-cell line derived neurotrophic factor (GDNF) is a potent survival factor for motor neurons. Our preliminary data show that transgenic overexpression of GDNF in developing muscle leads to dramatic hyperinnervation at motor endplates. This effect is specific for GDNF as overexpression of several neurotrophins also known to promote motorneuron survival does not influence the number of axons converging at the neuromuscular junction. In five specific aims we now propose to explore the mechanism by which GDNF causes hyperinnervation, ask whether long-term excess of GDNF can permanently prevent naturally-occurring synapse elimination, and test the hypothesis that GDNF (or a GDNF family member) regulates synapse elimination during normal development. To accomplish these aims, we have developed new techniques including an intracellular injection method for introducing foreign genes into single muscle fibers in vivo and a method of transplanting muscle from null mice into normal hosts. Determining how target-derived trophic factors influence synaptic competition on a local level will advance our understanding of the stability and plasticity of synaptic circuits. The dramatic effects of GDNF on muscle fiber innervation suggest a role for this molecule in the treatment of denervating conditions such as motor neuron diseases, neuropathies, and trauma.